1. Field of the Invention
This invention relates to a process for preparing cyanato group containing phenolic resins. More particularly, this invention relates to a process of preparing such resins which have improved properties.
2. Prior Art
Phenolic resins are a class of synthetic materials that have grown continuously in terms of volume and applications for over several decades. The building blocks used in greatest volume are phenol and formaldehyde. Other important phenolic starting materials are the alkyl-substituted phenols, including cresols, xylenols, p-tert-butyl-phenol, p-phenylphenol, and nonylphenol. Diphenols, e.g., resorcinol (1,3-benzenediol) and bisphenol-A [bis-A or 2,2-bis(4-hydroxylphenyl)propane], are employed in smaller quantities for applications requiring special properties. In addition to formaldehyde, acetyldehyde or furfuraldehyde sometimes are employed but in much smaller quantities. The greater latitude in molecular structure, which is provided by varying the raw materials, chemistry, and manufacturing process, has made possible an extremely large number of applications for these products as a result of the array of physical properties that arise from the synthetic options.
The early investigation of the reaction of phenol and formaldehyde began with the work of von Baeyer and others in the early 1870's as an extension of phenol based dye chemistry. The initial experiments result in soluble, amorphous products whose properties elicited little interest. Insoluble, cross-linked products also were reported in the late 1880's, but these products also were not perceived as useful materials. In 1888, the first patent for a phenolic-resin product intended for use as a hard-rubber substitute was granted. The first commercial product was introduced as a shellac substitute by the Louis Bluner Company in the early 1900's. Process patents were issued in 1894 and 1895 for ortho- and para-methylolphenol, respectively.
Key innovations in early phenolic-resin manufacture included control of the molecular structure and the use of heat and pressure to achieve desirable physical properties in filled compositions. Studies in the use of acidic or basic catalysts and of changes in the molar ratio of formaldehyde to phenol resulted in the definition of two classes of polymeric materials which are referred to as Bakelite resins. Caustic-catalyzed products, which are prepared with greater than a 1:1 mol ratio of formaldehyde to phenol, can be used to form cross-linked, insoluble, and infusible compositions in a controlled fashion. With less than a 1:1 mol ratio of formaldehyde to phenol, the resultant products remain soluble; furthermore, acid catalysis yields permanently stable compositions, whereas base-catalyzed materials can be advanced in molecular weight and viscosity. Possibly of greatest importance to early commercialization, however, was the reduction to practice of the use of heat and pressure to produce essentially void-free molding compositions.
Resole resins are made with an alkaline catalyst and a molar excess of formaldehyde. Novolak or novolac resins are prepared with an acid catalyst and less than one mol of formaldehyde per mol of phenol. The initial reaction involved in the preparation of resolated novolacs is carried out with an acid catalyst and less than a 1:1 mol ratio of formaldehyde to phenol. After formation of the novolac, the pH is adjusted so that the reaction mixture is basic and additional formaldehyde is added. Resoles and resolated novolaks are inherently thermosetting and require no curing agent for advancement. Novolacs, by comparison, are thermoplastic and require the addition of a curing agent, the most common being either hexamethylene-tetramine or a resole. The stages of molecular weight advancement are characterized by liquid or solid phenolic polymer which is soluble in certain organic solvents and is fusible; solid resin which is insoluble but swelled by organic solvents and, although softened by heat, exhibits essentially no flow; and an insoluble, infusible product which is not swelled by solvents nor softened by heat, i.e., the system is in a highly cross-linked state.
Phenolic resins have many uses. For example, such materials are used as bonding agents in friction materials such as brake linings, clutch facings, transmission bonds and the like. For example, U.S. Pat. Nos. 4,268,157; 4,069,108; 4,268,657; 4,218,361; 4,219,452; and 3,966,670 describe various friction materials in which a phenolic resin is employed as the bonding agent. Phenolics are also used as molding materials, and as coatings and adhesives. Phenolic resins developed for non-flammability and long-term temperature stability to 230.degree. C. have been studied in carbon-fiber composites. Potential for such composites lies in advanced aircraft application.
While present day phenolics exhibit several beneficial properties, they suffer from a number of disadvantages which restrict their utility. For example, such materials exhibit less than desirable thermal oxidative stability. Other major problems of present day phenolic technology include a need for auxiliary chemicals such as hexamethylene-tetramine to crosslink the phenolic which often results in the production of volatile by-products such as ammonia during crosslinking is often extensive and is not controllable.
Various modifications to phenolics have been proposed to obviate certain of the disadvantages attendant to these resins. For example, epichlorohydrin has been reacted with the hydroxyl groups of novolak forming epoxy novolak. Moreover, n-chloro-2-propene has been reacted with the hydroxyl groups of novolac to form the corresponding form methylon resin. Illustrative of other modified phenolics are those described in U.S. Pat. Nos. 4,650,838; 4,650,839; 4,757,118; and 4,771,113 to Das et al.
Japanese Patent Publications Nos. 59-149918, and 58-34822 describe a method of preparing a phenolic resin containing cyanate groups. In this method, a trialkyl ammonium salt of a phenol novolak is reacted with excess cyano halogen in an organic solvent such as methylene chloride. The ammonium by-product salt is separated from the reaction mixture by extraction with water. Several disadvantages are attendant to the process of these references. For example, the process is suitable only for cyanation of low molecular weight novolac resin below 450 Mn. The method disclosed in these references results in a phenolic cyanate resin which release smoke (volatiles) during curing at 155.degree. C. or above.
U.S. Pat. No. 3,448,079 describes aromatic cyanic acid esters produced by the reaction of phenolic resins with cyanogen halide in which the hydroxyl groups of the phenol-formaldehyde resins are replaced with cyanic acid ester groups, and process for producing same. U.S. Pat. No. 3,444,137 describes curable phenol-aldehyde resins characterized by molecules which contain a cyano group, an amine nitrogen atom, a phenyl group and a substituted hydroxyl group, such molecules having been made by reacting a phenol, formaldehyde and a cyano substituted primary or secondary amine. U.S. Pat. No. 4,022,755 describes cyanato-group containing phenol resins, and a process for preparing same. U.S. Pat. No. 4,713,442 discloses a polytriazine which comprises 1, 3, 5-triaryloxytriazines. Polyaromatic cyanates are also disclosed in EPA 0147548, WO85/03713 and GB-A-1218447.
Cyanato group containing phenolic resins have been described in Delano et al., Synthesis of Improved Phenolic Resins, Acurex Corp/Aerotherm, Acurex Vinyl Report 79-25/AS, Sep. 4, 1979 prepared for NASA Lewis Research Center, Contract No. Nas3-21368, and is available through the United States Department of Commerce National Technical Information Service.
A recent reference, Heat Resistance Polymers by Critchley et al., pp. 406-408, Plenum Press, New York, 1986, has described phenolic triazine resins prepared from phenolic novolac or meta-cresol novolac which have essentially the same chemical structures as described in the above referenced patents.
The phenolic-triazines which have been disclosed have been found to have high thermal stability. However, they have not been commercially produced because of poor shelf life, and a gel time too short for processing using conventional plastic processing equipment. It has been found as illustrated below, that reproduction of the phenolic cyanate ester resins disclosed in the art are unstable and not suitable for commercial applications such as matrix for various composites, impregnation media for paper and nonwovens, adhesives, coatings, molding compositions and the like. When these unstable resins are converted into a crosslinked product (phenolic-triazines) mechanical properties have been observed to be poor. The cured resins are so brittle, that frequently a suitable test sample for property determination cannot be fabricated. It has been found that curing the phenolic cyanate ester resins prepared according to the disclosures in the art, generates smoke and volatile chemicals.
U.S. Pat. No. 4,831,086 disclose a new class of phenolic cyanates and phenolic-triazine resins. The phenolic cyanate resins are disclosed to be stable as measured by gel time. The phenolic triazine resins are disclosed to be thermally stable as measured by Thermal Gravimetric Analysis. U.S. Pat. No. 4,831,086 also discloses an improved method for making cyanato-containing phenolic resins. In this method the cyanato-containing phenolic resin is formed by reacting an uncyanated phenolic resin, referred to as "phenolic resin", such as novolac resin, and a base, preferably trialkylamine in a cyclic ether solvent to form the corresponding trialkylammonium salt of novolac at room temperature. The trialkylammonium salt is then reacted with a cyanogen halide in the cyclic ether solvent to form the cyanato-containing phenolic resin. It is particularly preferred to conduct the reaction at a temperature below about -5.degree. C. and preferably from -5.degree. C. to -45.degree. C., more preferably -5.degree. C. to -30.degree. C., and most preferably -15.degree. C. to -30.degree. C. The reaction product is purified at a temperature of from 0.degree. C. to -45.degree. C. preferably by precipitation in a nonsolvent such as an alcohol, preferably isopropanol.